This is a great example of a fast moving target. Under Blog, the Oxford English Dictionary says "see Weblog". The second definition of the noun Weblog is: "A frequently updated web site consisting of personal observations, excerpts from other sources, etc., typically run by a single person, and usually with hyperlinks to other sites; an online journal or diary." Their definition of the verb Blog is "To write or maintain a weblog. Also: to read or browse through weblogs, esp. habitually."

But clearly the term "Blog" covers a vast area of communication. See the results for the Google search: define: blog. In the Google Directory, the search "blog directories" gets 126 hits, many of which seem to be private companies' blogs or their directories of other companies' blogs.

You can get a sense of how varied "Blogs" are, by doing a Google web search for: "neuroscience blog" site:edu, which gets abut 36 unique hits out of 163. Many of these are extremely technical interchanges among experts in the field; collaborations on research projects, etc. To make things even more complicated, see the 16 (of about 106) hits for the Google web search: "difference between blogs and wikis" site:edu. Clearly, we do not yet have a neat definition for this relatively recent and rapidly evolving phenomenon.

The Online Dictionary for Library and Information Science may get about as close as we are likely to get: "Weblog: A Web page that provides frequent continuing publication of Web links and/or comments on a specific topic or subject (broad or narrow in scope), often in the form of short entries arranged in reverse chronological order, the most recently added piece of information appearing first. An example in the field of library and information science is LISNews.com, which accepts postings from its readers... Also spelled Web log. Synonymous with blog. The process of maintaining a Weblog is known as blogging".